(core) Exposing custom widgets on the UI

Summary:
Exposing custom widgets as a dropdown menu in custom section configuration panel.

Adding new environmental variable GRIST_WIDGET_LIST_URL that points to a
json file with an array of available widgets. When not present, custom widget menu is
hidden, exposing only Custom URL option.

Available widget list can be fetched from:
https://github.com/gristlabs/grist-widget/releases/download/latest/manifest.json

Test Plan: New tests, and updated old ones.

Reviewers: paulfitz, dsagal

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3127

app/client/components/CursorMonitor.ts

@@ -52,7 +52,9 @@ export class CursorMonitor extends Disposable {
     this.autoDispose(doc.cursorPosition.addListener(pos => {
       // if current position is not restored yet, don't change it
       if (!this._restored) { return; }
-      if (pos) { this._storePosition(pos); }
+      // store position only when we have valid rowId
+      // for some views (like CustomView) cursor position might not reflect actual row
+      if (pos && pos.rowId !== undefined) { this._storePosition(pos); }
     }));
   }
 

app/client/components/ViewConfigTab.js

@@ -771,20 +771,7 @@ ViewConfigTab.prototype._buildCustomTypeItems = function() {
   }, {
 
     // 2)
-    showObs: () => activeSection().customDef.mode() === "url",
-    buildDom: () => kd.scope(activeSection, ({customDef}) => dom('div',
-      kf.row(18, kf.text(customDef.url, {placeholder: "Full URL of webpage to show"}, dom.testId('ViewConfigTab_url'))),
-      kf.row(5, "Access", 13, dom(kf.select(customDef.access, ['none', 'read table', 'full']), dom.testId('ViewConfigTab_customView_access'))),
-      kf.helpRow('none: widget has no access to document.',
-                 kd.style('text-align', 'left'),
-                 kd.style('margin-top', '1.5rem')),
-      kf.helpRow('read table: widget can read the selected table.',
-                 kd.style('text-align', 'left'),
-                 kd.style('margin-top', '1.5rem')),
-      kf.helpRow('full: widget can read, modify, and copy the document.',
-                 kd.style('text-align', 'left'),
-                 kd.style('margin-top', '1.5rem'))
-    )),
+    // TODO: refactor this part, Custom Widget moved to separate file.
   }, {
 
     // 3)